In today's optical networks, e.g., Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET), a GMPLS signaling is used to establish a bi-directional LSP, which comprises a forward path and a reverse path, wherein the forward path is established on “downstream on-demand”, i.e., the downstream node is allocated and labeled only on request of the upstream node; the reverse path is established on “downstream initiative”, i.e., the downstream node provides the label to the upstream mode initiatively. The bi-directional LSP comprises two LSPs with the same entry and exit nodes but opposite direction, the LSP from entry to exit node corresponds to the downstream label; while the LSP from exit to entry node corresponds to the upstream label. When establishing the bi-directional LSP, usually the upstream node designates upstream label for the reverse path of the bi-directional LSP, and the upstream label is designed to establish reverse connection. For instance, in the case of bi-directional multiplexing segment protection ring shown in FIG. 1, there are node A, B, C, D, . . . on the ring; suppose a bi-directional LSP is to be established between node A and D, and the available link label resource is shown in Table 1 (the links in Table 1 refer to bi-directional links between nodes), the available label set includes available upstream label set and available downstream label set.
TABLE 1LinkAvailable Upstream Label SetAvailable Downstream Label SetA-B1, 3, 41, 2, 3, 4B-C2, 3, 42, 3, 4C-D2, 43, 4
Due to the fact that the time slot (i.e., label) occupation on the bi-directional multiplexing segment protection ring should be consistent, the process of establishing a bi-directional LSP from node A to D according to traditional method is as follows (shown in FIG. 2):
First, the first node A initializes a request to establish a bi-directional LSP by sending a signaling message carrying upstream label 1 designated from the upstream label set (may be designated randomly or orderly) and available downstream label set {1, 2, 3, 4} to node B in step 1; node B receives the bi-directional LSP establishing request from node A, verifies the availability of upstream label 1, and then establishes a signaling message of requesting for establishing a bi-directional LSP (the signaling message carries the upstream label 1 and available downstream label set {2, 3, 4} of its link segment) and forwards the message to node C in step 2; node C receives the signaling message from node B and finds that the upstream label 1 is unavailable (i.e., mismatch with available upstream label set from node B to Node C), and determines the LSP can't be established, in step 3, node C sends a rejection message to node B, which in turn sends the message to the first node A in step 4, and returns the label set {2, 3, 4} acceptable by the current link segment to the first node A. The first node A receives the rejection message, chooses upstream label 3 from available upstream label set from the first node A to node B according to the returned acceptable label set {2, 3, 4}, and then initiates a signaling message of requesting for establishing a bi-directional LSP (the signaling message carries upstream label 3 and available downstream label set {1, 2, 3, 4}) to node B in step 5; node B receives the signaling message and finds upstream label 3 is acceptable, and then forwards the signaling message carrying upstream label 3 and available downstream label set {2, 3, 4} for its current link segment to node C in step 6; node C receives the signaling message and finds the upstream label 3 is available for itself, and then forwards the upstream label 3 and available downstream label set {3, 4} for current link segment to node D through establishing a signaling message of request for establishing a bi-directional LSP; node D receives the message forwarded from node C and finds the upstream tag 3 is unavailable (i.e., mismatch with the available label set from node C to node D), and then returns a rejection message and an acceptable label set {2, 4} for current link segment to node C in step 8, through node C to node B in step 9, and through node B to the first node A in step 10. Node A receives the rejection message, chooses upstream label 4 from the available upstream label set from node A to node B according to the returned acceptable label set {2, 4}, and initiates a bi-directional LSP establishing request in step 11, 12, and 13, node B and C receive the bi-directional LSP establishing request in step 11, 12 and 13 in sequence and find the upstream label 4 is acceptable, and then node C forwards the request to node D so that the reverse LSP of the bi-directional LSP is established. At the same time, node D chooses the downstream label 3 according to the available downstream label set {3, 4} and returns a successfully-established message and downstream label 3 to node C in step 14, through node C to node B in step 15, and through node B to node A in step 16; the first node A confirms the message, thus the bi-directional LSP is established.
Seen from above example, retrials of upstream label exists because node A doesn't take into account the restriction of acceptable labels for subsequent nodes while designating the upstream label; in the extreme case, the number of retrials is equal to the number of nodes involved minus 1. The existence of label retrials has a strong impact on the establishing time of GMPLS-based bi-directional LSP in SDH/SONET networks, and it even more delays service restoration at fault recovery. Therefore, the success ratio of traditional method is very low in the case that the available labels on the link where the labeled switching path passes are limited.